EXO3
Session assets
Discussion on Slack
Oral and Poster presentations and abstracts
Water-rich planets should be ubiquitous in the universe. Among the current exoplanet populations, many of those worlds are subject to important irradiation from their host star. As a consequence, water-rich worlds display supercritical water layer surrounded by an extended steam atmosphere making them good candidates for matching the observed mass-radius distribution of sub-Neptunes [1]. Here we describe a model that computes a realistic structure for water-rich planets by combining an interior model with an updated equation of state (EoS) for water, and an atmospheric model that takes into account radiative transfer. Our model has been applied to the GJ 9827 system as a test case and indicates Earth- or Venus-like interiors for planets b and c, respectively. Planet d could be an irradiated ocean planet with a water mass fraction of ∼20 ± 10%. We also provide mass-radius relationships for water-rich planets and their analytical expression. This allows one to directly retrieve a wide range of planetary compositions, without the requirement to run the model. The possible existence of such planets is discussed in light of atmospheric loss processes, suggesting that some sub-Neptunes are the outcome of planets that lost their H/He reservoirs.
Figure 1. Mass-radius relationships produced by our model (green, yellow and red thick lines) [2], compared to mass-radius relationships of planets with only condensed phases and no atmosphere (black, grey and light blue thin lines) [3,4]. A few planets of the solar system, the GJ-9827 system and the TOI-178 system are shown as well [5,6].
[1] Mousis, O., Deleuil, M., Aguichine, A., et al. 2020, ApJL, 896, L22.
[2] Aguichine, A., Mousis, O., Deleuil, M., et al. 2021, accepted in ApJ.
[3] Zeng, L., Sasselov, D. D., & Jacobsen, S. B. 2016, ApJ, 819, 127.
[4] Brugger, B., Mousis, O., Deleuil, M., et al. 2017, ApJ, 850, 93.
[5] Kosiarek, M. R., Berardo, D. A., Crossfield, I. J. M., et al. 2021, AJ, 161, 47.
[6] Leleu, A., Alibert, Y., Hara, N. C., et al. 2021, A&A, 649, A26.
How to cite: Aguichine, A., Mousis, O., Deleuil, M., and Marcq, E.: Modeling the structure of irradiated ocean planets - implications for mass-radius relationships, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-27, https://doi.org/10.5194/epsc2021-27, 2021.
The vigour and style of mantle convection in tidally-locked super-Earths may be substantially different from Earth's regime where the surface temperature is spatially uniform and sufficiently cold to drive downwellings into the mantle.
The thermal phase curve for super-Earth LHS 3844b suggests a solid surface and lack of a substantial atmosphere. The dayside temperature is around 1040 K and the nightside temperature is around 0 K, which is unlike any temperature contrast observed at present day for planets in the Solar System. On the other hand, the thermal phase curve of super-Earth 55 Cnc e suggests much hotter temperatures with a nightside temperature around 1380 K and a substellar point temperature around 2700 K. The substellar point is also substantially shifted eastwards, which requires efficient energy circulation in the atmosphere.
Here, we use constraints from thermal phase curve observations to model the interior mantle flow. To constrain the surface temperature of 55 Cnc e, we use the results from general circulation models varying the atmospheric composition and optical depth.
Depending on how strong the surface temperature contrast is, this can lead to hemispheric tectonic regimes. Such a regime could influence a planet's atmosphere through interior-exterior coupling mechanisms (e.g. volcanic outgassing).
We run geodynamic simulations of the interior mantle flow using the mantle convection code StagYY. The models are fully compressible with an Arrhenius-type viscosity law where the mantle is modelled with an upper mantle, a perovskite-layer and a post-perovskite layer. The lithospheric strength is modelled through a plastic yielding criteria and the heating mode is either basal heating only or mixed heating (basal and internal heating).
For LHS 3844b we find that the surface temperature dichotomy can lead to a hemispheric tectonic regime depending on the strength of the lithosphere and the heating mode in the mantle. In a hemispheric tectonic regime, downwellings occur preferentially on one side and upwellings rise on the other side (Fig. A). We compare these results to the case of 55 Cnc e, where large parts of the surface could be molten. At first order we expect that a magma ocean could homogenise the temperatures at the interface between the magma ocean and the underlying solid mantle and therefore reduce the likelihood of hemispheric tectonics operating on 55 Cnc e (Fig. B).
For LHS 3844b, the contribution of the interior flux to the thermal phase curve is on the order of 15-30 K, and therefore below the detecting capabilities of current and near-future observations. However, for hemispheric tectonics, upwellings might lead to preferential melt generation and outgassing on one hemisphere that could manifest as a secondary signal in phase curve observations. Such signals could also be produced on hotter planets such as 55 Cnc e where parts of the surface are hot enough to melt.
How to cite: Meier, T. G., Bower, D. J., Lichtenberg, T., Tackley, P. J., and Demory, B.-O.: Exploring the convection in super-Earths: Comparing LHS 3844b with 55 Cnc e, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-500, https://doi.org/10.5194/epsc2021-500, 2021.
Exoplanet atmospheres are often treated as a single monolithic whole in transmission spectroscopy, while in reality they are dynamic and complex objects rich with inhomogeneities. We will show that using the technique of transit limb scanning it is possible to recover additional information on the spatial, chemical and velocity distributions of planet atmospheres that would otherwise have been lost.
We will present results of transit limb scanning applied to a variety of hot jupiters, in particular using high resolution data of the sodium absorption to recover the equatorial and polar wind speeds on several planets spanning the temperature regime, and a new model of the metastable helium outflow of WASP 107b, where we find strong evidence of an extended comet-like tail.
In each case we fit a variety of models with different atmosphere architecture in increasing complexity to the time resolved high resolution transmission spectra, using a Bayesian evidence approach to model comparison in order to find the model that best explained the data.
Mapping exoplanets will be the key to solving a number of theoretical issues, as atmospheric wind speed is governed by the level of drag in the atmosphere, which is not well constrained by theoretical models. Similarly, the shape of helium outflows on evaporating planets will be important to proper determinations of evaporation rate, which sculpts the shape of the entire planet population.
How to cite: Louden, T.: Mapping exoplanet atmospheres in high energy environments, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-726, https://doi.org/10.5194/epsc2021-726, 2021.
The hot Jupiter exoplanet population is extremely diverse, spanning more than an order of magnitude in instellation, gravity, rotation period and atmospheric metallicity. As a new era of high-quality exoplanet observation fast approaches, with the coming James Webb Space telescope and Ariel missions providing wide wavelength coverage and ground-based telescopes supplying high resolution data, hot Jupiters will continue to be the best targets for atmospheric characterisation. A hierarchy of atmospheric models, from 1D radiative transfer codes coupled with advanced chemical networks up to 3D GCMs with complex treatment for atmospheric dynamics, have been used to model these planets, often trading modelling complexity for computational speed and cost. Yet, most of these planets are tidally locked and their atmospheres are intrinsically 3D, so they cannot be fully understood with 1D or 2D models. For example, predictions for the water feature strength observed in the HST/WFC3 bandpass vary drastically between 1D and 3D models, see Figure 1. This is increasingly prevalent at higher temperatures where strong day-side temperature inversions caused by TiO/VO may result in the feature being observed more strongly in emission than 1D models currently predict.
Figure 1: Comparison between our 3D grid computed using the non-grey SPARC/MiTgcm, a 1D grid of models using the 1D radiativ/convectivee transfer code CHiMERA and observation data.
Calculated using the state-of-the-art non-grey global circulation model SPARC/MiTgcm, we here create the largest library of 3D hot Jupiter models yet. This consists of 150 models spanning a wide range of instellation, metallicity, gravity, rotation period and the presence of strong photo-absorbing molecules TiO and VO, with the view to additionally explore planetary radius and atmospheric drag in the future. In addition to systematically varying these parameters, we also vary them jointly, something which has not been incorporated in any previous studies to date. From these simulations, we analyse the atmospheric pressure-temperature profiles and dynamic flows to identify any resulting qualitative trends in the atmospheric properties. We also calculate secondary eclipse spectra, transit spectra and phase curves to investigate how the day/night heat redistribution and spectral properties vary with planetary parameters. From this analysis we observe huge variations in the atmospheric properties within the grid. For example, the redistribution factor, an expression of day to night heat redistribution, varies by up to a factor of two depending on the parameters in our grid, see Figure 2. This means that, when working in synergy, intrinsic planetary parameters may cause divergence from previously computed models by a factor greater than any other additional processes such as nightside clouds, chemistry or MHD effects. Additionally, degeneracies in these values for our grid parameters, for instance higher surface gravity models having similar redistribution factors to higher metallicity, low surface gravity, models, complicates the picture even further.
Ultimately, we will interpolate this grid of GCM simulations to compare specific predictions for our range of possible observed planets, and compare with observed and predicted trends in hot Jupiter observations.
Figure 2: Comparison of redistribution factor, defined as f= (Tday/Teq)4, between models in our grid. Here we show the effects of surface gravity and metallicity. A value of 1 on this plot equates to an idealised fully homogenous atmosphere and a value of 2.66 to a case with no day to night heat redistribution.
How to cite: Roth, A. and Parmentier, V.: Exploring Hot Jupiter Atmospheres with a Grid of 150 Parameterised Non-Grey GCM Simulations., Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-744, https://doi.org/10.5194/epsc2021-744, 2021.
The hot Jupiter HD 80606b has an extremely eccentric orbit (e ~ 0.93). The stellar irradiation the planet received drastically increases by more than two orders of magnitude during the perihelion passage. The variation due to eccentric orbits provides unique opportunities to directly probe the dynamical, radiative, and chemical timescales. To understand the interplay between these processes, we set up a model framework to explore the atmospheric response. We first run three-dimensional general circulation models of HD 80606b using MITgcm [1] for various atmospheric metallicities and internal heat. Based on the hemispheric-averaged results from the GCM, we then time-step the 1D photochemical model VULCAN [2] along the orbits to investigate the compositional response in detail. The kinetics results show that efficient vertical mixing leads to deep quench levels of CO and CH4. Because of the depth of quench levels where the radiative timescale is long, the temperature variations have no effects on the quenching behavior. Instead, photolysis and the rapid heating in the upper stratosphere is the main driver of the time-dependent chemistry. We find a transient state of [CO]/[CH4] > 1 after the perihelion. Since the quenched abundances of CO and CH4 are independent of the orbital phase, the [CO]/[CH4] ratio can provide constraints on the metallicity and internal heat. In addition, we find a sequence of sulfur species initiated by the sudden thermal and photochemical forcing near perihelion, which can be potential signatures for the existence of sulfur on hot Jupiters.
Figure 1. Dayside averaged temperature as a function of time relative to perihelion from the 3D general circulation model, assuming solar metallicity and Tint =100 K. The white dotted line indicates perihelion and the black dotted lines enclose the period where the planet is in synchronous rotation.
Figure 2. The dayside-averaged CO mixing ratio as a function of time relative to perihelion (left), compared to the hypothetical scenario where the planet is fixed at each orbital location (right) to isolate the orbital effects.
How to cite: Tsai, S.-M., Steinrueck, M., Parmentier, V., Lewis, N., and Pierrehumbert, R.: Compositional Variations of the Highly Eccentric Planet HD 80606b, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-684, https://doi.org/10.5194/epsc2021-684, 2021.
Introduction
During the past few years, exoplanet atmospheric studies have grown tremendously. Detections of atomic and molecular species are reported from the ultraviolet to the infrared at low- and high-resolution for Earth-mass to Jupiter-mass planets and from temperate to ultra-hot worlds. Nowadays, ground-based high-resolution spectrographs are starting to produce groundbreaking results that inform us of the temperature structure and dynamics of exoplanet atmospheres.
A reference system for atmospheric studies: WASP-127b
WASP-127b (Lam et al. 2017) orbits a bright (V = 10.17) G5-type star with a period of 4.18 days. The host star is at the end of its main sequence phase with an age of ~10Gyr and radius of 1.30RS and has entered the sub-giant phase. Therefore, the planet might be going through an inflation process for the second time (Lopez & Fortney 2016), leading to its large radius (~1.31RJ). WASP-127b is a lukewarm (~600 times Earth irradiation) Saturn at the right mass border of the evaporation desert. Assuming a hydrogen-helium composition, its atmospheric scale height is about 2100 km. In this case, the signal in transmission for one scale height is around 420 ppm, potentially making this planet one of the best among those of its class to study exo-atmospheres through transmission spectroscopy.
Near-infrared low-resolution data obtained with the Hubble Space Telescope (Spake et al. 2020) have revealed the strongest water band amplitude in an exoplanet with a mean amplitude of about 800ppm. Moreover, the presence of clouds and hazes is necessary to explain the complete spectrum. Such planets with a high-amplitude water signature in the J-band are well placed to be studied from the ground with visible and NIR spectrographs, allowing the impact of hazes and clouds to be analyzed by measuring the water content at different wavelengths.
Observations of WASP-127b with ESPRESSO
ESPRESSO is a fiber-fed, ultra-stabilized high-resolution echelle spectrograph installed at Paranal. It can collect the light from each 8m Unit Telescope (UT) individually or the 4UTs simultaneously of the Very Large Telescope (VLT). Two transits of WASP-127b were obtained within the framework of the GTO consortium. We published our results in Allart et al. 2020, which includes the first measurement of spin-orbit alignment of this system revealing the peculiar orbital architecture of the system. The old star WASP-127 is a slow rotator while its planet has a misaligned retrograde orbit (Fig. 2). This is surprising as stars with Teff below 6250K are supposed to have aligned systems. An explanation could be that WASP-127b remained trapped in a Kozai resonance with an outer companion and only recently migrated close to its star.
A view on the atmosphere of WASP-127b
We used the ESPRESSO data to analyze the transmission spectrum of the exoplanet. Visible high-resolution datasets are known to reveal the exoplanet thermosphere through atomic species as the well-known sodium doublet. The WASP-127b transit datasets have revealed an excess of absorption of 0.34 +/- 0.04% at the expected position of the sodium doublet corresponding to a small extension over 7 scale heights. However, we do not detect the presence of other atomic species but we were able to set upper limits of only a few scale heights.
We also undertake a thorough search for water vapor at visible wavelengths. To do so, we used the well-known cross-correlation function technique to average thousands of water lines in velocity space. To be as model-independent as possible, we applied binary masks containing the position of water lines at different temperatures and with an increasing number of lines. We showcased that such a technique is sensitive enough to retrieve planetary signals by doing an injection recovery test. Once applied to the transit datasets of WASP-127b, we were able to put a 3-sigma upper limit on the presence of water vapor at visible wavelengths of only 38ppm on the average depth of the 1600 strongest water lines at the equilibrium temperature. This upper limit corresponds to an atmospheric extension of only 0.08 scale height.
Combining low- and high-resolution spectroscopy
Based on this unique precision obtained with ESPRESSO, we explored the compatibility between ground-based high-resolution spectroscopy and space-based low-resolution spectroscopy. We computed a grid of models with different grey cloud-deck pressure based on the results of Spake et al. 2020 and using the pi-eta line-by-line radiative transfer code (Pino et al 2018). On one hand, models with grey cloud deck pressure ranging from 0.3 to 1 mbar are compatible with the HST datasets, while in another hand only models with pressure below 0.5 mbar are compatible with the ESPRESSO datasets. Therefore, by combining the low- and high-resolution datasets of WASP-127b, we can constrain the grey cloud pressure between 0.3 and 0.5 mbar.
Conclusion
We used two transits of WASP-127b obtained with ESPRESSO to acquire a thorough understanding of the orbital architecture of the system and the exoplanet atmosphere.
We propose a new framework to search for water vapor and other molecular species at high resolution. Despite the good data quality, we did not detect water vapor. However, we combined this result with a low-resolution detection of water at 1.3 microns to constrain the presence of clouds in the atmosphere of WASP-127b.
To conclude, we report for the first time that high-resolution visible data can be used to differentiate between cloudy and cloud-free exoplanets by measuring the water content and can also provide essential information on the cloud-deck pressure. The framework developed here to measure this water content will be applied to other exoplanets in the ESPRESSO GTO atmospheric survey and other surveys such as the NIRPS GTO atmospheric survey.
References
Allart R., Pino, L., Lovis, C. et al., 2020, A&A, 644, A155
Pino, L., Ehrenreich, D., Wyttenbach, A., et al. 2018, A&A, 612, A53
Spake, J. J., Sing, D. K., Wakeford, H. R., et al. 2020, MNRAS
How to cite: Allart, R. and the The ESPRESSO consortium: WASP-127b: a misaligned planet with a partly cloudy atmosphereand tenuous sodium signature seen by ESPRESSO, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-438, https://doi.org/10.5194/epsc2021-438, 2021.
1. Introduction
Brown dwarfs are object between giant planets and star in term of mass, numerous observations suggest active meteorology. A near-infrared brightness variability is observed among L and T dwarfs. Several atmospheric dynamics have been proposed, but the mechanisms remain unclear. Clouds could also play an important role in shaping the thermal structure and spectral properties of these atmospheres via their opacity. In this study we propose to use a 3D convection resolving model couple to a grey-band radiative transfer and to a microphysical to study radiative cloud feedback over a large set of temperature, cloud composition and metallicity. The mechanism discussed in this study could play a role in the observed flux variability in brown dwarfs and is also applicable for directly imaged extrasolar giant planets.
2. Model
To study these mechanisms, the 3D non-hydrostatic dynamical CM1 is used coupled to a grey band radiative transfer [1]. An idealized cloud microphysical model was added, considering MgSiO3 , Fe, Al2O3 , CaTiO3 , Cr and MnS particles [3, 4, 5]. Particle settling is included. The effect of metallicity is taken into account. The clouds are radiatively active using Rosseland mean opacities. The cloud particles are considered spherical following a gaussian size distribution, with radius between 0.01 and 100 μm. The density of cloud particle is a free parameter. The model is initialized using temperature profile from a 1D model [2].
Figure 1: Initial vertical temperature profile (solid line) and condensation profile for the considered clouds (dotted line).
3. Results
The model shows that the convective layer height increases in function of the temperature (Fig 2). An estimate of the vertical mixing is determined. With the inclusion of radiative property of the clouds, some feedback is observed. MgSiO3 , Fe and Al2O3 clouds tend to expend the convective layer altitude. This feedback is most visible at high temperature and depends on the clouds particle size. CaTiO3 , Cr and MnS have very low impact on the convective layer, due to a thin cloud layer and a low abundance.
Figure 2: Vertical profile of the mean potential temperature (K) for the different temperature cases considerate without clouds.
References
[1] Freedman, R. S., et al. The Astrophysical Journal SupplementSeries, 214(2):25. 2014.
[2] Tan, X. and Showman, A. P. The Astrophysical Journal, 874(2):111. 2019.
[3] Visscher, C., et al. The Astrophysical Journal, 716(2):1060–1075. 2010.
[4] Wakeford, H. R., et al. Monthly Notices of the Royal Astronomical Society, 464(4):4247–4254. 2014.
[5] Morley, C. V., et al. The Astrophysical Journal, 756(2):172. 2012.
How to cite: Lefèvre, M., Tan, X., Lee, E., and Pierrehumbert, R.: Cloud-convection feedback in brown dwarfs atmosphere, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-675, https://doi.org/10.5194/epsc2021-675, 2021.
Abstract
The Nancy Grace Roman Space Telescope, to be launched in 2025, will directly image exoplanets in reflected starlight for the first time. This observing technique will enable the study of a population of exoplanets whose atmospheres cannot be characterized with current techniques. Previous works have analysed the possible science outcome of reflected-starlight observations through atmospheric retrievals. These have shown that an accurate atmospheric characterization from a reflected-light spectrum will likely be hindered if the radius and the atmospheric properties (e.g. cloud properties) of the exoplanet are unknown a priori. In this work we study how different observing strategies can improve the atmospheric characterization in such cases. We conclude that combining measurements at different star-planet-observer phase angles (α) is a powerful strategy to better characterize the atmosphere of an exoplanet and constrain its radius.
Introduction
Understanding how planetary and atmospheric properties affect the reflected-starlight spectra of exoplanets is a key to interpreting future direct-imaging observations. This may also help propose observing strategies to optimize the prospects for characterization of the planet. That understanding of the physical fundamentals of exoplanet remote sensing will be applicable to space missions such as the Roman Telescope, LUVOIR or HabEx.
Model and retrieval procedure
We assume a H2-He atmosphere with methane as the only absorbing species and a cloud layer. The atmospheric parameters that we consider in our model are: the methane abundance (fCH4), the optical thickness of the cloud (τc), the altitude at which the cloud top is located, the geometrical extension of the cloud, the single-scattering albedo (ω0) of the aerosols and their effective radius (reff). We also include the planet radius (Rp) as another model parameter. Through Mie theory, reff determines the aerosol scattering phase function which describes the interaction between the photons and the aerosols depending on the incident direction of the light.
For a grid of ~300,000 atmospheric configurations, we computed their reflected-starlight spectra (500-900 nm) at phase angles α=37º, 85º and 123º by means of a pre-existing multiple-scattering radiative transfer code[1]. The spectral resolution is R~125-225, consistent with mission concepts like LUVOIR or HabEx. We simulated observations of three atmospheric configurations, labelled as cloud-free, thin-cloud and thick-cloud scenarios. These observations were simulated at each of the three phase angles considered by adding wavelength-independent noise. The signal-to-noise ratio was set to S/N=10 in all cases.
We used the MCMC retrieval method developed in Carrión-González et al. (2020)[2] to run atmospheric retrievals for each of the single-phase simulated observations at α=37º, 85º and 123º. For that, we assumed no prior knowledge of any of the model parameters. Furthermore, we extended this retrieval method in order to run simultaneous retrievals of multiple observations at different phase angles.
Results
We first ran single-phase retrievals (Fig. 1) at each phase angle and repeated this for the three cloud scenarios. We verify that no single-phase observation with S/N=10 allows us to distinguish between cloudy and cloud-free atmospheres. This is due to the parameter correlations between Rp, the cloud properties and the methane abundance. This consistent with the findings in Carrión-González et al. (2020) for α=0º. We also find that a single-phase observation at a large phase angle (123º) can constrain the planet radius with an error smaller than 35% in all of the cloud scenarios. Our findings are consistent with previous works carrying out single-phase retrievals of reflected-starlight spectra[3].
We also ran multi-phase retrievals to combine spectra obtained at different phase angles. We tested the combinations 37º+85º, 37+123º and 37º+85º+123º. For comparison, we also ran single-phase retrievals at α=37º with an increased signal-to-noise ratio of S/N=20. The adopted phase angles are within the ranges of observable conditions for the known exoplanets that will be detectable by LUVOIR and HabEx. Such ranges of α might also be observable for certain targets in an optimistic configuration of the Roman Telescope coronagraph[4].
We find that combining small (37º) and large (123º) phase angles is an effective strategy to break some of the aforementioned correlations between model parameters (Fig. 1). Both of the combinations (37+123º) and (37º+85º+123º) allow to distinguish between cloudy and cloud-free atmospheres in all cloud scenarios and accurately constrain most of the model parameters. Combining small (37º) and moderate (85º) phase angles fails to achieve this. Similarly, a single-phase retrieval at α=37º with S/N=20 also fails to break the parameter correlations.
The shape of the aerosol scattering phase function is found to affect the improvements in multi-phase retrievals. We verified that the improvements are smaller for more isotropic scattering phase functions. The cloud properties and their uncertainties hence play an important role in the retrievals. Indeed, we find that if the cloud properties are assumed known a priori, the retrieval results become overly optimistic. This is consistent with previous works adopting such assumptions[5] and happens because less correlations between model parameters take place in the retrievals.
We therefore conclude that combining small and large phase angles is a generally effective observing strategy to accurately characterize exoplanets for which no prior information on the planet radius or cloud composition is available. This suggests that exoplanets with wide ranges of observable phase angles should be priority targets for reflected-starlight telescopes.
References
[1] García Muñoz & Mills (2015), A&A, 573, A72
[2] Carrión-González et al. (2020), A&A, 640, A136
[3] Nayak et al. (2017), PASP, 129, 973
[4] Carrión-González et al. (2021), A&A, accepted
[5] Damiano et al. (2020), AJ, 160, 206
How to cite: Carrión-González, Ó., García Muñoz, A., Santos, N. C., Cabrera, J., Csizmadia, S., and Rauer, H.: Reflected-starlight phase curves: an observing strategy to constrain the radius and atmospheric properties of directly imaged exoplanets, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-694, https://doi.org/10.5194/epsc2021-694, 2021.
Summary
We quantify the potential of a mid-infrared space-based mission like LIFE (Large Interferometer for Exoplanets) [1, 2] to detect atmospheric signatures of life in the spectrum of a terrestrial planet at various stages of its evolution. We use a Bayesian retrieval framework to simulate observations obtained through LIFE. Using the minimal requirements for the spectral resolution, the signal to noise ratio, and the wavelength range for LIFE, as discussed in Konrad et al. [3] (a contribution to EPSC 2020), we retrieve the atmospheric structure and composition of thermal spectra of the Earth at various epochs [4]: modern day, the Neoproterozoic Oxygenation Event (NOE, 0.8 billion years ago), the Great Oxygenation Event (GOE, 2.0 billion years ago), and a prebiotic Earth (3.9 billion years ago).
These simulations determine in more detail the potential of the LIFE space mission. They also further constrain the technical requirements of the mission, as well as determining the robustness of the analyses routines.
Context
Temperate terrestrial exoplanets are predicted to be very abundant in our galaxy [5]. Such planets are the preferred targets to study when searching for life in the universe.
The only way to characterize a terrestrial exoplanet in the context of its habitability is by studying its atmosphere. Atmospheric spectra are strictly linked to the chemical composition, the temperature structure of the atmosphere, the presence of clouds, the emission and scattering from the surface, as well as many other processes.
With the current instruments, it’s hard to find planets that are similar to the Earth in the habitable zone around their stars. Because of the lack of observational data, we rely on theoretical spectra of terrestrial exoplanets to develop analysis pipelines that could be most effective for the characterisation of such targets. In addition to that, a theoretical approach can be used to determine the technical requirements for future missions, in order to be able to detect the most relevant atmospheric signatures.
Space missions that aim at characterizing terrestrial exoplanets have been proposed, e.g. HabEx [6] and LUVOIR [7] which focus on the reflected (visible and near-infrared) portion of the planetary spectrum, and LIFE [1, 2], the Large Interferometer for Exoplanets, which will characterize terrestrial planets in the thermal (mid-infrared), emitted portion of the planetary spectrum.
Using nulling interferometry, LIFE will allow us to further constrain the bulk parameters and the surface conditions of a few dozens of terrestrial planets, as well as to gather information about their atmospheric structure and composition. The current status of this project, as well as the summary of all the submitted contributions, is discussed in more detail in Angerhausen et al. (2021, this conference).
Methods
Bayesian retrieval routines are, so far, the key to a statistically significant analysis of an atmospheric spectrum. These have been successfully used in other astrophysical contexts, as well as similar atmospheric analyses of Jovian exoplanets [see e.g. 8].
The Bayesian retrieval routine that is currently used is composed by two modules:
- the forward model petitRADTRANS [9], which applies the radiative transfer equation to produce theoretical spectra given a set of parameters (pressure-temperature structure, chemical abundances, bulk parameters,…);
- the parameter estimation module, which applies the Bayesian Inference Nested Sampling method [10] to assess the goodness of the theoretical spectrum when compared to the observed spectrum.
We validated the routine with an Earth twin orbiting a Sun-like star at 10 pc distance from the observer, as presented in Konrad et al. [3]. In our previous work, we used petitRADTRANS both to produce the input spectrum and as forward model for the Bayesian routine. While still being useful to determine the minimum requirements of the LIFE Mission with respect to resolution, signal to noise ratio and wavelength range, this approach is very simplified and prone to bias.
In this work, to better assess the robustness of the routine with different input spectra, we use instead simulated spectra of the Earth at various stages of its evolution calculated by Rugheimer & Kaltenegger [4]: a prebiotic Earth (at 3.9 billion years ago, or Ga), the Earth during the Great Oxygenation Event at 2.0 Ga and the Neoproterozoic Oxygenation Event at 0.8 Ga, and the modern Earth. We consider both cloudfree and cloudy atmospheres.
We simulate observations obtained with LIFE by using its simulator LIFESim (Ottiger et al., in prep.), assuming these four scenarios and considering an Earth-sized on a 1 AU orbit around a Sun-like star at 10 pc distance. LIFESim allows to calculate the wavelength-dependent S/N considering the major astrophysical and instrumental noise sources (e.g. stellar leakage, local zodiacal dust, exozodiacal dust).
We consider a spectral resolution of 50, a signal to noise ratio of 10, and a wavelength range of 4-18.5 micron, which are the minimum requirements for LIFE as discussed in Konrad et al. [3].
Results
As shown in Figures from 1 to 4, the simulated spectra of the Earth [3] in the various epochs appear very different. The presence of a 60% global cloud cover also has an impact on the shape of the spectrum.
Branching out from the modern Earth-twin scenario is therefore essential to understand to what extent LIFE would be able to differentiate between the various evolutionary stages of a terrestrial planet, both before and after the appearance of life on the surface.
Retrieval runs are currently ongoing. Some preliminary results can be seen in Figure 5, which shows the retrieved spectra of a cloudfree prebiotic Earth planet and of a cloudfree modern Earth, compared to the input spectra (from [3]). The retrieval suite appears able to reproduce the main features of the spectrum, with subsequent insight on the atmospheric compositions and structures.
A thorough analysis of these retrievals will be discussed in more detail and presented in this conference.
How to cite: Alei, E., Konrad, B., Mollière, P., Quanz, S., and Angerhausen, D. and the LIFE Collaboration: Diagnostic potential of the mid-infrared space interferometer LIFE for studying Earth analogues, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-340, https://doi.org/10.5194/epsc2021-340, 2021.
Context
Current knowledge suggests that temperate terrestrial exoplanets commonly occur in our galaxy (Bryson et al., 2021). One long-term goal of exoplanet research is to characterize the atmospheres of a sizable sample of temperate planets. Such measurements will add to our knowledge of the diversity of worlds in our galaxy, which is a driving force behind this effort, and they will enable the discovery of habitable worlds.
The sensitivity of currently planned space- and ground-based observatories will unlikely allow for the characterization of a large number of temperate exoplanet atmospheres. Concepts for future missions (e.g. NASA's LUVOIR [a] and HabEx [b]), capable of characterizing exoplanet atmospheres in reflected light (optical and near-infrared (NIR) range), have been proposed. Complementary to these efforts, the LIFE mission introduced in Quanz et al. (2018) (see also Angerhausen et al. (2021, this conference) for a general overview) is a proposed space-based nulling interferometer which measures the mid-infrared (MIR) thermal emission of exoplanets.
The MIR regime is particularly powerful, since it provides probes for atmospheric molecules not accessible in the optical or NIR (e.g. the biosignatures CH4 and N2O). Furthermore, MIR emission observations, in contrast to optical and NIR observations, allow us to probe the temperature structure, radius and surface conditions on exoplanets (Quanz et al., 2019; Konrad et al. in prep.). Previous work has focussed on constraining technical requirements for LIFE via retrievals of cloud-free Earth-twin MIR spectra (Konrad et al., 2020; Konrad et al. in prep.). We now generalize these estimates by looking beyond the cloud-free Earth-twin. Specifically we consider Venus (this submission) and Earth’s evolution through time (Alei et al. (2021, this conference)).
Methods
Our approach is based on the Bayesian retrieval of simulated mock observations with LIFE for a Venus-twin exoplanet around a sun-like star located at 10 pc. We generate Venus’ emission spectrum using the 1D radiative transfer tool petitRADTRANS (Mollière et al. 2019). The model takes the planetary radius and mass, the pressure-temperature (PT) profile, the atmospheric composition and cloud properties as input. We parametrize Venus’ PT profile via a 3rd order polynomial and consider line and collision-induced absorption by H2O, CO2, and CO as well as absorption and emission contributions from Venus’ opaque H2SO4 clouds.
With LIFESim(Ottiger et al. in prep.), we simulate the wavelength-dependent SNR expected for observations with LIFE for a Venus-twin. LIFESim considers noise contributions from stellar leakage, local zodiacal dust, and exozodiacal dust.
Our retrieval mechanism is built upon the Multinest algorithm (Feroz et al., 2009). Using the LIFESim noise estimate as uncertainty on the Venus spectrum, we retrieve for the mass and radius, the PT profile, the surface pressure, the molecular abundances and the cloud parameters. By covering a range of different wavelength ranges (3-20 μm, 6-17 μm), spectral resolutions (R = 20, 35, 50, 100) and signal to noise ratios (S/N = 5, 10, 15, 20), we obtain estimates for the technical requirements LIFE needs to meet to:
- Differentiate between a Venus- and an Earth-twin.
- Find evidence for clouds in Venus’ atmosphere.
- Accurately characterize structure and composition of Venus’ atmosphere.
This approach will show whether the previously determined technical requirements for LIFE (Konrad et al., in prep.) are sufficient. Many similar MIR retrieval studies have been performed for Earth-like exoplanets. However, to the best of our knowledge, no MIR retrieval studies exist for a Venus-like planet.
Results
In Figures 1 and 2, we provide first preliminary results, as a proof of concept from testing our approach for retrieving cloudy MIR spectra with our retrieval framework. We used a Venus-twin spectrum (range = 3 - 20 μm, R = 50, SNR = 20) assuming a distance of 10 pc from the Sun as input for this retrieval run. Please note, that for these retrieval validation runs, we only consider the planet’s photon noise and not yet additional noise sources.
Figure 1 shows the spectrum corresponding to the retrieved parameters in relation to the input spectrum. From this plot we see that our retrieval provides a good fit to the input spectrum. Figure 2 shows that the PT profile parameters are well retrieved for the upper atmosphere of Venus. Due to the opaque cloud layer in Venus’ atmosphere at approx. 0.1 bar, the surface temperature and surface pressure are no longer retrievable (as can be seen from the cut in the green uncertainty at 0.1 bar). Instead we interpret the retrieved “surface” pressure as the pressure at which Venus’ cloudy atmosphere becomes opaque.
With these first retrieval runs we demonstrate that our retrieval suite produces reliable results for cloudy atmospheres. Further studies are currently carried out and will be presented at the conference. Furthermore, we will address the points listed above and discuss how results from cloudy retrievals could be safely interpreted.
References
Quanz, S. P., et al., 2018, Proc. SPIE, 107011I
Bryson, S. et al., 2021, AJ, 161, 36. doi:10.3847/1538-3881/abc418
Quanz, S. P. et al., 2019, arXiv e-prints arXiv:1908.01316
Konrad, B. S., et al., 2020, European Planetary Science Congress
Mollière, P., et al., 2019, A&A, 627:A67
Feroz, F., et al., 2009, MNRAS, 398(4):1601–1614
Websites
[a] https://www.luvoirtelescope.org
[b] https://www.jpl.nasa.gov/habex/
How to cite: Konrad, B. S., Alei, E., Angerhausen, D., and Quanz, S. P. and the LIFE collaboration: Atmospheric Retrieval of Cloudy Venus-Twin Exoplanets in the Context of the LIFE Mission, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-578, https://doi.org/10.5194/epsc2021-578, 2021.
The next generation of astronomical facilities will be able to retrieve exoplanetary atmospheric spectra in increasing quantity and of increasing quality. Radiative transfer (RT) models of these atmospheres is essential both for interpreting observational data and for linking these data to the planetary physical state with the aid of dedicated climate models. So far, a large effort has been placed in modelling the atmospheres of giant planets, which are the most easily accessible to observations. Now times are ripe to extend these studies to treat the relatively thin atmospheres of terrestrial-type exoplanets, which are the most likely targets for the search of atmospheric biomarkers.
Here we present a procedure to perform radiative transfer calculations for terrestrial-type exoplanets with temperate physical conditions (Simonetti et al. in preparation). The procedure is based on HELIOS and HELIOS-K, which are novel, flexible and publicly available codes developed by the University of Bern (Grimm & Heng, 2015; Malik et al., 2017, 2019; Grimm et al., 2021) as a part of the Exoclime Simulation Platform (ESP) repository. These codes make full use of the computing power of Graphics Processing Units (GPUs, colloquially known as graphics cards) being therefore much faster (up to one order of magnitude, see Grimm et al. 2021) than other similar codes and are integrated with a variety of molecular and atomic line repositories such as HITRAN (Gordon et al., 2017), HITEMP (Rothman et al., 2010) and Kurucz. Until now, HELIOS has been mostly applied to study Jupiter-like planets. The main features of the procedure that we have implemented for the treatment of rocky, habitable planets can be summarized as follows.
First, we added the treatment of the continuum absorption features of a variety of gases, in particular H2O (Clough et al., 1989; Mlawer et al., 2012) and CO2 (Gruszka & Borysow, 1997; Baranov et al., 2004; Baranov, 2018). These continua strongly contribute to the overall opacity of Earth-like atmospheres and cannot be neglected. Second, we paid special attention to the sub-Lorentzian profile of CO2 absorption lines, testing the effects of different recipes (Perrin & Hartmann, 1989; Tonkov et al., 1996). Third, we considered different hypotheses regarding the convective lapse rate of the troposphere. On these basis we: (i) tested the robustness of HELIOS and HELIOS-K against changes in model variables and (ii) compared them with other codes already published and used in the same context (e.g. LBLRTM Clough et al., 2005), as done by Yang et al. (2016).
One of the main goals of this work is to provide a new and fast radiative transfer treatment for the ESTM, an energy balance climate model with upgraded treatment of the vertical and horizontal energy transport Vladilo et al. (2015). The ESTM is extremely flexible and allows for a rapid exploration of the planetary and atmospheric parameter space, providing us the ability to map quantitative indices of habitability on these parameters (Silva et al., 2017). The flexibility of both HELIOS and ESTM will allow us to test a wide variety of atmospheric compositions, which have applications in the study both of exoplanets and of ancient Earth and Mars. Moreover, the HELIOS procedure adapted to terrestrial-type atmospheres can be used to generate synthetic TOA fluxes useful to link the conditions at the planet's surface with quantities that will become observable with future generations of instruments, such as secondary eclipse spectra and direct IR emission spectra from terrestrial-type exoplanets (see e.g. Quanz et al., 2021). Finally, the output of the same procedure can be applied to other codes in the ESP repository, such as the THOR GCM (Mendonca et al., 2016; Deitrick et al., 2020).
Figure 1 shows the TOA albedo obtained for three different stellar spectral classes for different values of surface temperature and stellar zenith angle, for an atmosphere of 1 bar of CO2 and a relative H2O humidity of 100%, as obtained by HELIOS using the procedure presented in Simonetti et al. (in preparation). The surface albedo was set to 0.15.
Figure 2 shows the relation between OLR and surface temperature for different radiative transfer models for an Earth-like atmosphere composed by N2, O2, 360 ppmv of CO2, 1.8 ppmv of CH4 and a temperature-dependent quantity of H2O (relative humidity equal to 100%). The thick red curve labelled "HELIOS" has been obtained with the novel procedure presented in Simonetti et al. (in preparation). The data relative to the other curves have been taken from Yang et al. (2016).
REFERENCES:
Baranov, Y. I. 2018, Journal of Molecular Spectroscopy, 345, 11
Baranov, Y. I., La erty, W. J., & Fraser, G. T. 2004, Journal of Molecular Spectroscopy, 228, 432
Clough, S. A., Kneizys, F. X., & Davies, R. W. 1989, Atmospheric Research, 23, 229
Clough, S. A., Shephard, M. W., Mlawer, E. J., et al. 2005, JQSRT, 91, 233
Deitrick, R., Mendonca, J. M., Schro enegger, U., et al. 2020, ApJS, 248, 30
Gordon, I. E., Rothman, L. S., Hill, C., et al. 2017, JQSRT, 203, 3
Grimm, S. L., & Heng, K. 2015, ApJ, 808, 182
Grimm, S. L., Malik, M., Kitzmann, D., et al. 2021, ApJS, 253, 30
Gruszka, M., & Borysow, A. 1997, Icarus, 129, 172
Malik, M., Kitzmann, D., Mendonca, J. M., et al. 2019, AJ, 157, 170
Malik, M., Grosheintz, L., Mendonca, J. M., et al. 2017, AJ, 153, 56
Mendonca, J. M., Grimm, S. L., Grosheintz, L., & Heng, K. 2016, ApJ, 829, 115
Mlawer, E. J., Payne, V. H., Moncet, J. L., et al. 2012, Philosophical Transactions of the Royal Society of London Series A, 370, 2520
Perrin, M. Y., & Hartmann, J. M. 1989, JQSRT, 42, 311
Quanz, S. P., Ottiger, M., Fontanet, E., et al. 2021, arXiv e-prints, arXiv:2101.07500
Rothman, L. S., Gordon, I. E., Barber, R. J., et al. 2010, JQSRT, 111, 2139
Silva, L., Vladilo, G., Murante, G., & Provenzale, A. 2017, MNRAS, 470, 2270
Tonkov, M. V., Filippov, N. N., Bertsev, V. V., et al. 1996, Applied Optics, 35, 4863
Vladilo, G., Silva, L., Murante, G., Filippi, L., & Provenzale, A. 2015, ApJ, 804, 50
Yang, J., Leconte, J., Wolf, E. T., et al. 2016, ApJ, 826, 222
How to cite: Simonetti, P., Vladilo, G., Malik, M., Silva, L., Ivanovski, S., Maris, M., Murante, G., Biasiotti, L., Bisesi, E., and Monai, S.: Terrestrial-type planetary atmospheres with HELIOS, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-819, https://doi.org/10.5194/epsc2021-819, 2021.
Sub-Neptunes (Rp~1.25-4 REarth) remain the most commonly detected exoplanets to date. However, it remains difficult for observations to tell whether these intermediate-sized exoplanets have surfaces and where their surfaces are located. Here we propose that the abundances of trace species in the visible atmospheres of these sub-Neptunes can be used as proxies for determining the existence of surfaces and approximate surface conditions. As an example, we used a state-of-the-art photochemical model to simulate the atmospheric evolution of K2-18b and investigate its final steady-state composition with surfaces located at different pressures levels (Psurf). We find the surface location has a significant impact on the atmospheric abundances of trace species, making them deviate significantly from their thermochemical equilibrium and “no-surface” conditions. This result arises primarily because the pressure-temperature conditions at the surface determine whether photochemically-produced species can be recycled back to their favored thermochemical-equilibrium forms and transported back to the upper atmosphere. For an assumed H2-rich atmosphere for K2-18b, we identify seven chemical species that are most sensitive to the existence of surfaces: ammonia (NH3), methane (CH4), hydrogen cyanide (HCN), acetylene (C2H2), ethane (C2H6), carbon monoxide (CO), and carbon dioxide (CO2). The ratio between the observed and the no-surface abundances of these species, can help distinguish the existence of a shallow surface (Psurf < 10 bar), an intermediate surface (10 bar < Psurf < 100 bar), and a deep surface (Psurf > 100 bar). This framework can be applied together with future observations to other sub-Neptunes of interest.
Figure 1: Selected criteria and a flowchart of possible steps to identify the existence of the surface and the surface pressure for a hydrogen-dominated exoplanet with properties similar to K2-18b.
How to cite: Yu, X., Moses, J., Fortney, J., and Zhang, X.: How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-184, https://doi.org/10.5194/epsc2021-184, 2021.
1. Introduction
Hydrogen emisssion by degassing magma is likely to have a significant impact on the H2 mixing ratio in early atmospheres of terrestrial planets depending on the surface pressure, volcanic flux and oxygen fugacity of the magma [1]. This process can account for the presence of a few percent of hydrogen in the early atmospheres of terrestrial exoplanets.
The formation of water using a N2-CO2-H2 mixture was identified in a previous experimental study focusing on the early Earth with nitrogen as a dominant gas and an initial H2 mixing ratio fixed at 4% [2]. We now explore various amounts of hydrogen in the mixture to cover the unknown parameters regarding surface emission and to highlight the role played by hydrogen in the chemistry of these exoplanetary atmospheres.
2. Experimental method
The PAMPRE (french for “aerosol production in microgravity using a reactive plasma”) experimental setup [3] is a plasma reactor used to simulate the photochemistry at low pressure (around 1 hPa) in planetary atmospheres. The CO2-N2-H2 mixture is injected in the reactor with various N2 to H2 abundance ratio and a fixed abundance of 70% for carbon dioxide. The molecular hydrogen mixing ratio is varied from 5% down to 0.5% in volume. The gas phase in the PAMPRE reactor is analyzed using a quadrupole mass spectrometer (Hiden Analyticals) to identify the species formed once the plasma is turned on.
3. Experimental results
Figure 1. Production of water and oxygen with 0.5% H2
Figure 2. Production of water and oxygen with 5% H2
The significant oxygen production associated to a hydrogen ratio of 0.5% is reduced drastically when increasing the hydrogen mixing ratio by a factor of 10. The formation of oxygen via the HOx cycle well known in the case of Mars becomes secondary when the hydrogen mixing ratio increases. As a result, the production rate of the OH radical formed by (1) increases.
O(1D) + H2 --> OH + H (1)
The radical OH then reacts by (2) and (3) which explains the significant formation of water with an initial hydrogen mixing ratio of 5%.
H2 + OH --> H2O + H (2)
2 OH --> H2O + O (3)
These new experiments put forward the presence of a hydrogen mixing ratio threshold over which the formation of water dominates the formation of oxygen. Two regimes can therefore be identified : O2-dominant production regime at low amounts of H2 and H2O-dominant production regime at higher amounts of H2.
4. Conclusions and perspectives
For terrestrial exoplanets known to be in the habitable zone of their host star, the present study shows that molecular hydrogen in the atmosphere is critical to characterize their habitability as this particular molecule leads to an important formation of water in oxydized conditions. It is also shown that based on the hydrogen abundance in the atmosphere, two regimes can be identified and this particular process suggests that high amounts of hydrogen could lead to a significant abundance of water in these atmospheres even without liquid water at the surface. Quantification of relative concentrations using quadrupole mass spectrometry by a recently developped tool [4] is in progress and would allow to quantify the H2 mixing ratio which seperates the two regimes in our experimental conditions. Based on these experimental results, the impact of this process is now being studied using the chemical kinetics model VULCAN [5] taking into account vertical mixing and using a known star spectrum. The predictions obtained from the model will be used to predict future observations of these specific environments in the frame of JWST and ARIEL space missions.
Acknowledgements
NC acknowledges the financial support of the European Research Council (ERC Starting Grant PRIMCHEM, Grant agreement no. 636829).
References
[1] Liggins, P., Shorttle, O., Rimmer, P., 2020. Can volcanism build hydrogen-rich early atmospheres ? Earth and Planetary Science Letters 550
[2] Fleury, B., Carrasco, N., Marcq, E., Vettier, L., Määttänen, A., 2015. The Astrophysical Journal Letters 807
[3] Szopa, C., Cernogora, G., Boufendi, L., Correia, J., Coll, P., 2006. Planetary and Space Science 54
[4] Gautier, T., Serigano, J., Bourgalais, J., Hörst, S.M., Trainer, M.G., 2020. Decomposition of electron ionization mass spectra for space application using a Monte-Carlo approach. Rapid Communications in Mass Spectrometry
[5] Tsai, S-M., Lyons, J.R., Grosheintz, L, Rimmer, P.B., Kitzmann, D., Heng, K.,2017. VULCAN : An Open-Source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres. The Astrophysical Journal Supplement Series 228
How to cite: Drant, T., Carrasco, N., Perrin, Z., Vettier, L., Gautier, T., Tsai, S.-M., and Heng, K.: Molecular hydrogen in oxidized atmospheres of terrestrial exoplanets : Implications for water and oxygen formation, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-503, https://doi.org/10.5194/epsc2021-503, 2021.
Abstract
In the perspective of detecting and characterizing more and more close-in-orbit hot terrestrial exoplanets transiting nearby M-dwarf stars (e.g., best Earth-size targets for observational studies), the atmosphere of our "sister" planet Venus is one of the most relevant cases to address observational prospects. The detection of Venus-like atmospheres at various orbital periods and planetary masses will create a picture of where the divergence between Earth and Venus originates [1].
In this work, we propose to use a 3D Global Climate Model (GCM) developed explicitly for exoplanets and paleoclimate studies [2] to simulate the atmospheres of a possible class of Venus-like exoplanets i.e., rocky, highly irradiated exoplanets orbiting M-dwarf stars. The objectives are twofold: 1) to study the impact of changing specific planetary parameters, e.g., planetary radius and gravity, in the large-scale atmospheric circulation; and 2) to reproduce synthetic emission and reflected phase curves capable of addressing observational prospects.
1.Introduction
In the Solar System, Venus and the Earth are examples of two rocky planets with similar radii and mass values and possibly a similar bulk composition [3]. Nonetheless, the two planets also represent a scenario of divergent climate evolution. Thus, in terms of mass and radius parameters, Venus and Earth would be astrophysically indistinguishable in the event we would be observing them as exoplanets.
Several modelling studies [4] established possible limits for both the habitable zone and the “Venus Zone” [1]. These 1D modelling studies offered a first approximation for the future differentiation between Earth-like and Venus-like exoplanet populations. Nonetheless, the complete characterisation and differentiation between these two populations can only be currently accomplished with dedicated 3D GCMs, which can reproduce the sort of climate feedbacks that ultimately constraint surface habitability conditions [5]. The diversity of planetary climates is expected to be significant given 1) the variety of planetary atmospheres on the rocky worlds of our Solar System 2) the diversity of the planetary parameters for the exoplanets observed so far. Thus, predicting the actual climate for a specific planet represents a challenging task [6].
A new Era for characterising rocky exoplanet atmosphere will be opened by the James Webb Space Telescope (JWST), foreseen to be launched in 2021 and continued by the ESA mission ARIEL scheduled for launch in 2029. In addition, a new generation of instruments is being developed, including a whole set of new high-resolution spectrographs for the Very Large Telescopes (VLT) and Extremely Large Telescopes (ELT).
2.GCM Modelling of Venus analogues
We propose using the Generic GCM firstly developed at the Laboratoire de Meteorologie Dynamique (hereinafter LMD G-GCM) to simulate Venus-like exoplanetary atmospheres and climate response to a realistic set of planetary parameters. We will also provide synthetic observables, reflected and emission phase curves, which are inherently 3D, to support the characterisation of the population of Venus-like exoplanets in the future.
The LMD G-GCM uses a 3D dynamical core, common to all terrestrial planets, and a planet-specific physical core. It includes an up-to-date generalised radiative transfer for variable gaseous atmospheric compositions made of various cocktails of CO2, N2, and H2O, O2, CO, using the correlated-k method. Processes such as the radiative effect of clouds or Rayleigh scattering are considered. For instance, it has successfully been used to simulate the climate of other exoplanets like the cold super-earth Gliese 581d, a tidally-locked exoplanet like Gliese 581c/HD85512b [2], or Proxima b [5].
In this work, the LMD G-GCM is adapted to study close-in Venus-like exoplanets orbiting M-dwarf stars. The work is focused on the response of large-scale atmospheric circulation to critical parameters: radius, gravity, surface atmospheric pressure, solid-body rotation rate. For the latter, we will be selecting two likely modes, 1:1 and 3:2 spin-orbit resonances. Each parameter will be changed while keeping the others fixed. Combinations of realistic parameter variations will be selected according to specific mass-radius relationships [7]. This will allow for the characterisation of temperature and wind fields at different pressure levels, particularly at and above the classical Venus top cloud layer.
3.Preliminary Results
To address the impact from simulating hot, dense atmospheres typical for Venus-like exoplanets, we run the LMD G-GCM using the planetary parameters of exoplanet TRAPPIST-1c as a framework for a possible Venus-like exoplanet. Synchronous rotation, with no obliquity and eccentricity, were assumed, together with a Venus-like atmosphere, with 92-bar surface pressure, and similar chemical composition with radiatively active Venus-type clouds, UV absorbers, and meridional variation of the cloud structure [8]. Quasi-convergence of temperature is achieved for the whole atmosphere after 15000 orbits (See Fig.1).
Using the last orbit, the quasi-steady state obtained will be taken as the initial state to run simulations varying the parameters space, allowing the study of atmospheric variables (See Fig.2). In addition, top-of-the-atmosphere longitude-latitude maps of outgoing fluxes computed by the LMD G-GCM will be used to produce phase curves and interpret possible variations due to atmospheric dynamics (e.g., superrotation, jets, waves).
Figure 1. Temperature profile (solid black line) for the quasi-steady state, used as a reference input for future simulations. The initial profile (thin blue line) is obtained after running the model for 300 days for a Venus-like planet orbiting a Sun-like star.
Figure 2. Instantaneous temperature field for the cloud-top pressure level (p~0.02-bar) for TRAPPIST-1c, 300 orbits after the quasi-steady state was obtained. The sub-stellar point is represented by the white star.
Acknowledgments
This work is supported by Fundação para a Ciência e a Tecnologia (FCT) through the research grants UIDB/04434/2020, UIDP/04434/2020, P-TUGA PTDC/FIS-AST/29942/2017.
References
[1] Kane et al.2018. ApJ.869
[2] Wordsworth et al.2011. ApJL. 733. L48.
[3] Hamano et al. 2013. Nature, 497, 607-610
[4] Kopparapu et al.2014. ApJL. 787.L29.
[5] Turbet et al.2016. A&A. 596. A112.
[6] Forget & Leconte 2014. Phil. Trans. R. Soc.A372.
[7] Zeng et al.2016. ApJ. 819. 127.
[8] Garate-López & Lebonnois 2018 Icarus 314.
How to cite: Gilli, G., Quirino, D., Navarro, T., and Turbet, M.: Planetary parameters impact in the large-scale circulation of Venus-like exoplanetary atmospheres and observational prospects, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-354, https://doi.org/10.5194/epsc2021-354, 2021.
Transmission spectroscopy provides us with information on the atmospheric properties at the limb, which is often intuitively assumed to be a narrow annulus around the planet. This is why all existing retrieval algorithms used so far to constrain the atmospheric composition from data rely either on i) a single 1D forward model, thus assuming a uniform limb or ii) a linear combination of 1D models to account for heterogeneities between different regions of the limb (e.g. east vs. west). Even full three-dimensional atmospheric models (GCMs) commonly use only the atmospheric
columns at the terminator to predict the observable transmission spectrum for a given simulation.
Here, we will present numerical experiment from hot Jupiters to ultra hot Jupiters which demonstrate that the region probed in transmission actually extends significantly toward the day and night sides of the planet and that, as a result, the real transmission spectrum computed from a GCM simulation with our fully 3D radiative transfer differs significantly from results obtained with the usual assumptions, especially for the hottest atmospheres. This comes from the fact that the terminator of hot, synchronously rotating planets is a region exhibiting sharp thermal and compositional gradients.
Fig 1: Sum up of the different geometry required in retrievals code to avoid biases according to the equilibrium temperature ofthe planet and the presence or the absence of optical absorber. 1D retrieval model appears to be correct for planets with equilibrium temperature lower than 1400 K when optical absorber (such as TiO, VO, Na, K, etc) are present in the atmosphere. They are howeverbiased above this limit where 2D retrieval code are mandatory.
Using realistic hot exo-atmospheres from GCM model SPARC/MIT, we will demonstrate how this effect can lead to strong biases is the temperature and abundances retrieved from actual data biases that will need to be addressed and corrected for if we want to be able to make robust inferences from future JWST and ARIEL data. We will determine a global limit where the biases due the 1D hypothesis of retrieval model start to be negligible. We have also established a hierarchy of the different 3D effects (vertical, horizontal along and/or across the limb) in order to be able to chosse the right tool to break down the biases (see Fig. 1). We will finally present our new tool, a 2D version of TauREx, which is designed to unravel these biases.
How to cite: Pluriel, W., Leconte, J., Zingalès, T., Falco, A., and Parmentier, V.: Evolution of the biases in retrieved atmospheric composition of hot Jupiters, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-5, https://doi.org/10.5194/epsc2021-5, 2021.
- 1Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
- 2SRON Netherlands Institute for Space Research, Utrecht, The Netherlands
- 3Centre for Exoplanet Science, University of Edinburgh, Edinburgh, UK
Introduction: Inferring physical and chemical properties of an exoplanet's atmosphere from its transmission spectrum is computationally expensive. A multitude of forward models, sampled from a high dimensional parameter space, need to be compared to the observation. The preferred sampling method is currently Nested Sampling [7], in particular, the MultiNest implementation [2, 3]. It typically requires tens to hundreds of thousands of forward models to converge. Therefore, simpler forward models are usually favoured over longer computation times.
A possible workaround is to use machine learning. A machine learning algorithm trained on a grid of forward models and parameter pairs can perform retrievals in seconds. This would make it possible to use complex models that take full advantage of future facilities e.g., JWST. Not only would retrievals of individual exoplanets become much faster, but it would also enable statistical studies of populations of exoplanets. It would also be a valuable tool for retrievability analyses, for example to assess the sensitivity of using different chemical networks.
The main obstacle to overcome is being able to predict accurate posterior distributions and error estimates on the retrieved parameters. These need to be as close as possible to their Bayesian counterparts.
Methods: Expanding on the 5-parameter grid in [5], we used ARCiS (ARtful modelling Code for exoplanet Science) [6] to generate a grid of 200,000 forward models described by the following parameters: isothermal temperature (T), planetary radius (RP), planetary mass (MP), abundances of water (H2O), ammonia (NH3) and hydrogen cyanide (HCN), and cloud top pressure (Pcloud). The models contain 13 wavelength bins, matching those of WASP-12b's observation with HST/WFC3 [4]. We added normally distributed random noise with σ=50 ppm.
We trained a random forest following the details in [5] and a convolutional neural network (CNN). We divided the data into a training set of 190,000 spectra and a test set of 10,000. For the CNN we reserved 19,000 spectra (10%) from the training set for validation. These are needed to update the network weights at each training iteration.
The CNN was trained with the loss function introduced in [1] to output a probability distribution. To account for the observational noise, we combined the distributions predicted for multiple noisy copies of the spectrum.
To evaluate the performance of the machine learning algorithms, we retrieved all the spectra in the test set and plotted our predictions against the true values for the parameters. We repeated the experiment with only 1,000 spectra for Nested Sampling, reflecting the increased computational overhead of each of these retrievals. We then used a transmission spectrum of WASP-12b observed with HST/WFC3 [4] as a real-world test case.
Results: Although the random forest trains faster, the CNN provided better results. Figures 1 and 2 show the predicted versus the true parameters for the CNN and Nested Sampling bulk retrievals. Remarkably, we observe the same structures in both plots. This shows that the CNN is able to learn the relationship between spectral features and parameters. We also found that both the CNN and Nested Sampling provide correct error estimates, with ~60% of predictions within 1σ of the true value, ~98% within 2σ, and virtually all within 3σ. This is in almost perfect agreement with expectation from statistical errors.
Figures 3 and 4 show the CNN and Nested Sampling retrieval of WASP-12b. Again, we see very similar results, although the CNN provides broader posterior distributions. Work is ongoing to try to fix this issue.
We found that a training set of 180,000 spectra is unnecessarily large, and the same performance can be reached with only 20,000 spectra. This implies that the number of forward model computations needed to train a CNN is smaller than the number needed for a single Bayesian retrieval. If this holds for more complex forward models and higher quality spectra, it would make machine learning an extremely attractive alternative to Nested Sampling.
Conclusion: The existing literature on machine learning retrievals of exoplanet atmospheres only has comparisons between machine learning and Nested Sampling for a handful of test cases [1, 5, 8]. In this work we present a comparison of bulk retrievals done with both methods, showing that machine learning can indeed be a viable and fast alternative to Nested Sampling. We are currently working on extending these results to models with equilibrium chemistry and to JWST/NIRSpec simulated spectra.
Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 860470.
References:
[1] Cobb, A. D., Himes, M. D., Soboczenski, F., Zorzan, S., O’Beirne, M. D., Baydin, A. G., Gal, Y., Domagal-Goldman, S. D., Arney, G. N., Angerhausen, D. (2019, 5). An Ensemble of Bayesian Neural Networks for Exoplanetary Atmospheric Retrieval.
[2] Feroz, F., Hobson, M. P. (2008). Monthly Notices of the Royal Astronomical Society 384 (2).
[3] Feroz, F., Hobson, M. P., Bridges, M. (2009). Monthly Notices of the Royal Astronomical Society 398(4).
[4] Kreidberg, L., Line, M. R., Bean, J. L., Stevenson, K. B., Desert, J.-M., Madhusudhan, N., Fortney, J. J., Barstow, J. K., Henry, G. W., Williamson, M. H., Showman, A. P. (2015). A DETECTION OF WATER IN THE TRANSMISSION SPECTRUM OF THE HOT JUPITER WASP-12b AND IMPLICATIONS FOR ITS ATMOSPHERIC COMPOSITION. Technical report.
[5] Marquez-Neila, P., Fisher, C., Sznitman, R., Heng, K. (2018). Supervised machine learning for analysing spectra of exoplanetary atmospheres.
[6] Min, M., Ormel, C. W., Chubb, K., Helling, C., Kawashima, Y. (2020). The ARCiS framework for exoplanet atmospheres. Astronomy & Astrophysics 642.
[7] Skilling, J. (2006). Nested sampling for general Bayesian computation. Bayesian Analysis 1 (4).
[8] Zingales, T., Waldmann, I. P. (2018). The Astronomical Journal 156 (6).
How to cite: Ardevol Martinez, F., Min, M., Kamp, I., and Palmer, P. I.: Machine learning as an ultra-fast alternative to Bayesian retrievals, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-713, https://doi.org/10.5194/epsc2021-713, 2021.
As next generation observation facilities become available for the characterisation of Exoplanet atmospheres, the capacity to move beyond the assumption of molecules existing in local thermodynamic equilibrium becomes more pressing. While atmospheric retrieval of Exoplanet atmospheres has operated under this assumption, it is known that many effects within planetary atmospheres drive molecules away from this equilibrium - into a state of non-local thermodynamic equilibrium, or non-LTE. Here we revisit the bi-temperature model for approximating non-LTE molecule populations which uses a two temperature parameterisation: distinct rotational and vibrational temperatures. Here the implementation of this model is used in the molecular cross section generation code Exocross (Yurchenko et al. 2018) to further explore the differences arising in forward modelled spectra due to molecules existing in non-LTE.
Now we include an exploration of non-LTE in the the optical with the additional consideration of Titanium Oxide (TiO) - this diatomic exhibits bands of accentuated intensity when in non-LTE. This can be seen in the figure shown here - forward model examples for TiO in the atmosphere of WASP-76b with 3 transit observations simulated for JWST's NIRSpec instrument set to use its G140H grating. Here the divergence between spectra for the LTE and non-LTE cases can be seen to be particularly pronounced around 0.85 micron.
In addition we show that the bi-temperature paramterisation is tractable for atmospheric retrieval. Here modifications were made to the publicly available atmospheric retrieval code TauREx 3 to include the additional temperature degree of freedom and non-LTE cross section data were generated using the Exocross code. This enables us to demonstrate the retrieval of both rotational and vibrational temperature parameters from JWST data simulated with under 10 transits.
How to cite: Wright, S. and Yurchenko, S.: Towards non-LTE retrieval of molecules in Exoplanet atmospheres, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-48, https://doi.org/10.5194/epsc2021-48, 2021.
The vast majority of the detected exoplanets orbit in less than a month around their star, in extreme conditions unmet in the Solar System. The demographics of these close-in planets exhibit a striking feature: the lack of Neptune-size worlds on very short orbits, also dubbed the "Neptunian desert", which challenges our understanding of planetary formation and evolution. Two classes of mechanisms thought to play a predominant role in shaping the desert are orbital migration, which brings such planets close to their star, and atmospheric escape under the resulting increased irradiation. Yet, their relative roles remain poorly assessed, in part because we lack numerical models that couple the two processes with high precision and on secular timescales.
To address this need, we developed a state-of-the-art model, the JADE code, which allows to self-consistently simulate the complete lifetime of a planet around its host star. On the dynamical side, the most impactful processes for close-in planets are implemented. The three-dimensional evolution of the orbit is modeled under stellar and planetary tidal forces, a relativistic correction, and the action of a distant perturbing body. On the atmospheric side, the vertical structure of the atmosphere is integrated over time based on its thermodynamical properties, composition, inner heating, and the evolving stellar irradiation, which results, in particular, in high-energy-induced photo-evaporation.
We bench-marked the JADE code on the intriguing case of GJ436 b, an evaporating Neptune at the fringes of the desert whose eccentric and misaligned orbit despite an advanced age is still a puzzle. We showed that its exciting properties can be naturally explained by a strong interplay between eventful orbital and atmospheric histories. Particularly, a hidden companion on a wide orbit is able to trigger a Kozai—Lidov resonance, trapping the inner planet in high-eccentricity cycles for secular periods of time. During this resonance phase, the atmosphere pulsates in tune with the Kozai—Lidov cycles, which leads to stronger tides and an earlier migration than predicted from pure dynamical simulations. Nonetheless, the planet still starts to evaporate upon migrating billions of years after its formation, refining the paradigm that mass loss is dominant in the early age of close-in planets.
Precise measurements of orbital and atmospheric tracers are crucial to constrain our models. Our joined approach to the understanding of past dynamical and atmospheric history, combined with the servicing of next-generation instruments, offers the best chance to shed light on the origins of the desert and the processes that forge the close-in planet population.
Article:
“The JADE code: Coupling secular exoplanetary dynamics and photo-evaporation”
O. Attia, V. Bourrier, P. Eggenberger, C. Mordasini, H. Beust, D. Ehrenreich
March 2021
Astronomy & Astrophysics, Volume 647, id.A40
DOI: 10.1051/0004-6361/202039452
Figure 1: Illustration of how the JADE code works. The evolution of the inner planet’s orbit as well as its atmospheric structure is monitored. The figure depicts the configuration of the system at two different secular time steps. It illustrates a typical case where the inner orbit shrank, circularized, and changed inclination, and where the envelope substantially eroded.
Figure 2: Possible interplay between secular dynamics and atmospheric evolution of GJ436 b, as unveiled by the JADE code. Top: a Kozai—Lidov resonance induced by a distant perturber generates large eccentricity oscillations. Middle: the periodic rise in eccentricity translates into shorter mean planet–star distance and higher stellar irradiation. This causes the atmosphere to heat up and pulsate in tune with Kozai—Lidov cycles (blue), as opposed to the case where this dynamical feedback is not taken into account (orange). Bottom: the increases in radius lead to stronger tidal effects and thus a faster migration (blue), as compared to the case where the atmosphere is not modeled (orange).
How to cite: Attia, O. and Bourrier, V.: Coupling the Atmospheric and Dynamical Evolution of Close-in Exoplanets, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-575, https://doi.org/10.5194/epsc2021-575, 2021.
1) Introduction
If an Earth-like planet with a large amount of water is drifted towards its host star, the surface temperature increases, which leads the atmosphere to enter a catastrophic runaway greenhouse state [1-4]. Studying this runaway greenhouse effect allows to better determine the runaway greenhouse insolation threshold and therefore the inner edge of the habitable zone (HZ).
Some studies [5-7] have shown that radiatively inactive background gases - such as N2 - can increase the Outgoing Longwave Radiation (OLR) and delay the onset of the runaway greenhouse. In other words, the OLR may “overshoot” the Simpson-Nakajima limit [4]. Consequently the inner edge of the HZ is moved closer to the star [8-11] and the planet is further from a catastrophic runaway greenhouse feedback. Explanations for this overshoot include the modification of the scale height of the atmosphere [7] or the coupled effect of the pressure broadening and a lapse rate close to a dry lapse rate [12]. However, there is still no consensus so far in the literature on whether an OLR overshoot is expected or not and what are the responsible mechanisms.
2) Method
First, we did multiple sensitivity tests to constrain the most important physical processes and parameterizations involved in 1D climate models by using a suite of models available in the literature [13-16]. This allows us to build a new hybrid 1D radiative-convective model named PyRADS-Conv1D to produce reference curves of the OLR relative to the surface temperature for H2O+N2 atmospheres (Fig. 1) to solve the question of the potential overshoot.
This sensitivity study, and particularly the understanding of the relevant processes involved in such atmospheres, are useful knowledge to produce accurate 3D simulations using a Global Climate Model (GCM). Therefore we are doing simulations with the LMD-Generic model for different theoretical study cases: a waterworld and a planet with Earth’s continents. Unique features of the LMD-Generic allow to explore the onset of the runaway greenhouse, even in situations where water is not a trace gas.
3) Results
By using the PyRADS-Conv1D model [17], we propose reference OLR curves relative to the surface temperature for different nitrogen pressures (Fig. 1). We confirm the occurrence of an overshoot of the OLR regarding the Simpson-Nakajima limit. We explain also that the non usual transition between a nitrogen dominated and a water dominated atmosphere challenges the modeling by making important usually second order processes. For example, neglecting the transition from the foreign to the self pressure broadening of the center of the water absorption lines leads to the suppression the overshoot [17]. PyRADS-Conv1D is based on the line-by-line radiative transfer method from PyRADS [7] but we built an accurate correlated-k table able to reproduce line-by-line results (exo_k curves on Fig. 1). This table is suitable for 3D GCM studies.
Finally we show through preliminary results from the LMD-Generic model (Fig. 2) that a GCM provides the same tendency than 1D models with a strong decrease of the OLR when the surface temperature increases, even without the radiative effect of the clouds. These results may be helpful to better understand the runaway greenhouse effect and thus the possible future of the Earth. We will explain in details during the presentation our results from 3D simulations for the different considered study cases assuming an H2O-dominated atmosphere.
Figure 1: OLR relative to the surface temperature for different nitrogen pressures. PyRADS-Conv1D is our reference 1D model. Exo_k [15] is an accurate 1D model used to test the correlated-k table built for this work.
Figure 2: OLR relative to the surface temperature with (blue curve) or without (green curve) the radiative effect of the clouds, using the 3D LMD-Generic model. Here we assume 1 bar of nitrogen.
References
[1] Komabayasi, M. 1967, Journal of the Meteorological Society of Japan. Ser. II
[2] Ingersoll, A. 1969
[3] Nakajima, S., Hayashi, Y.-Y., & Abe, Y. 1992, Journal of the Atmospheric Sciences
[4] Goldblatt, C. & Watson, A. J. 2012, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
[5] Goldblatt, C., Claire, M. W., Lenton, T. M., et al. 2009, Nature Geoscience
[6] Goldblatt, C., Robinson, T. D., Zahnle, K. J. et al., 2013, Nature Geoscience
[7] Koll, D. D. B. & Cronin, T. W. 2019, The Astrophysical Journal
[8] Leconte, J., Forget, F., Charnay, B. et al., 2013, Nature
[9] Kopparapu, R. k., Ramirez, R., Kasting, J. F., et al. 2013, The Astrophysical Journal
[10] Ramirez, R. M. 2020, Monthly Notices of the Royal Astronomical Society
[11] Zhang, Y. & Yang, J. 2020, The Astrophysical Journal
[12] Pierrehumbert, R. T. 2010, Principles of planetary climate (Cambridge ; New York: Cambridge University Press)
[13] Koll, D. D. B. & Cronin, T. W. 2018, Proceedings of the National Academy of Sciences
[14] Turbet, M., Ehrenreich, D., Lovis, C. et al. 2019, Astronomy & Astrophysics
[15] Leconte, J. 2021, Astronomy & Astrophysics
[16] Marcq, E., Salvador, A., Massol, H. et al. 2017, Journal of Geophysical Research
[17] Chaverot et al. 2021, In prep.
How to cite: Chaverot, G., Bolmont, E., and Turbet, M.: How background gases can delay the onset of the runaway greenhouse? Insights from 1D and 3D modeling., Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-759, https://doi.org/10.5194/epsc2021-759, 2021.